1. Field of Invention
This invention relates to a time-lag fuse having improved breaking capacity and is particularly related to such a fuse which has a unique double-tubular construction.
2. The Prior Art
It is well known that when an overload current, i.e., current is excess of the rated capacity, flows through a fuse, the fuse may explode resulting in fire hazard due to the drawing of a long arc between the terminals of the fusible wire element which is frequently maintained for a long time. The explosion is often so severe as to cause scattering of fragments of the fuse at a considerable distance, and when the arc is maintained the fuse terminals are destroyed.
It has long been recognized that the adverse effects and impact of the gases resulting from explosion of the fuse may be reduced by the use of so-called "arc-extinguishing" or "arc-quenching" materials. Thus, as disclosed in a very early patent (U.S. Pat. No. 550,638, issued Dec. 3, 1895), the fusible element is surrounded with a loose, porous, non-combustible material such as asbestos, in order to deaden the sound of explosion of the fusible element, prevent fragmentation and scattering, absorb the smoke and resulting gases and confine the destruction of the fusible element to a relatively small portion of its length.
The use of arc-quenching fillers is also mentioned in German Pat. No. 611,680.
More recent patents have also resorted to the use of arc-extinguishing materials for this purpose. For example, U.S. Pat. No. 3,069,520, issued on Dec. 18, 1962 to Cameron et al discloses the use of finely divided inert insulating material such as sand, disposed within the fuse holder and about the fusible element. U.S. Pat. No. 3,876,966, issued on Apr. 18, 1975 to Aloysuis J. Fister also discloses the use of "arc-quenching" filler such as quartz sand, for a similar purpose.
The use of arc-extinguishing fillers in the manner disclosed in the aforementioned illustrative patents to improve the breaking capacity of the fuse has had several disadvantages. As disclosed in the recent patent of Arikawa et al (U.S. Pat. No. 4,100,523, issued on July 11, 1978), it is difficult to stabilize the fuse characteristics when the fuse is filled with arc-extinguishing material as in the prior art. This is due to irregularities in the grain distribution of this material and variations in its filling rate. Moreover, a large amount of heat conduction from the fusible element to the arc-extinguishing material causes changes in the thermal balance of the fusible element and impairs the time-lag characteristic of the fuse and, at the same time, heat conduction from the fusible element to the arc-extinguishing material, and then to the fuse tube, tends to raise the temperature of the fuse tube excessively. These and other disadvantages impair the breaking capacity and time-lag properties of the fuse.
It is therefore an object of this invention to provide a fuse which has an improved breaking capacity and time-lag characteristics.
It is another object of this invention to provide a fuse with improved breaking capacity and time-lag characteristics which is free from the difficulties and disadvantages which are inherent in the prior art fuses employing arc-quenching materials.
The foregoing and other objects of this invention will be more clearly understood from the following detailed description of the invention, taken in conjunction with the accompanying drawing which is a longitudinal sectional view of a double-tubular fuse constructed in accordance with this invention.